Generally, exhaust gas emitted by internal combustion engines contains, for instance, particulate matter (PM) having carbon as a main component, and ash made up of unburned components, all of which are known to give rise to air pollution. Regulations concerning emissions of particulate matter have become stricter year by year, alongside regulations on harmful components such as hydrocarbons (HC), carbon monoxide (Co) and nitrogen oxides (NOx). Various technologies for trapping and removing particulate matter from exhaust gas have been therefore proposed.
For instance, particulate filters for trapping particulate matter are provided in the exhaust passage of internal combustion engines. In diesel engines, for example, a given amount of particulate matter is emitted together with the exhaust gas. Accordingly, a diesel particulate filter (DPF) is fitted in the exhaust passage. Such particulate filters include known filters, referred to as of wall flow-type, having a structure in which a substrate is made up of multiple porous cells, and in which the inlets and the outlets of the multiple cells are plugged alternately (PTL 1, 2). In a wall flow-type particulate filter, exhaust gas that flows in through cell inlets passes through a porous cell partition wall that separates the cells, and is discharged out through the cell outlets. As the exhaust gas passes through the porous cell partition wall, the particulate matter is trapped within the pores inside the partition wall.
Imparting NOx purification ability to the filter is an approach that has been studied in recent years with a view to further enhancing purification performance. For instance, it has been proposed to provide, in the filter, an SCR (Selective Catalytic Reduction) catalyst that selectively reduces NOx in the exhaust gas as a result of the reducing action of ammonia or the like (PTL 3). In an exhaust gas purification device of urea addition type, for instance, aqueous urea is supplied upstream of a filter that supports an SCR catalyst, whereupon ammonia is generated through hydrolysis of the aqueous urea. The ammonia adsorbs onto the filter, and NOx in the exhaust gas is purified through the reducing action of the adsorbed ammonia.